1. Field of the Invention
The present invention relates to a method for measuring stress exerted on an optical fiber, and in particular to a method for measuring stresses exerted on an optical fiber during manufacturing of the fiber or of a cabled fiber.
2. Technical Background
Numerous forms of fiber optic sensors have been developed to monitor parameters in various systems and processes, including the Fabry-Perot Interferometer, the Bragg Grating, the Mach-Zehnder Interferometer, and the Michelson Interferometer, to name but a few. These fiber optic sensors are used in a wide variety of applications, including use as strain gauges, dynamic pressure sensors, bearing condition sensors, non-contact proximity sensors, and temperature sensors. In each of these applications, the fiber optic sensor is fixedly attached to the system to be monitored, and usually is encased within a housing or rigid structure that is fixedly attached to the system to communicate parameter changes in the system to the fiber optic sensor.
As strain gauges, fiber optic sensors have been used to monitor dynamic strain. In such applications, the fiber optic sensor is imbedded within a material that is attached to a component of a structure such that the strain within the component may be monitored. Applications of fiber optic strain gauges have typically included civil structures such as dams, buildings, and bridges.
As dynamic pressure sensors, fiber optic sensors have been used in a variety of applications including the monitoring of performance of internal combustion engines, as well as monitoring the performance of compressors and pumps. When used to monitor the performance of an internal combustion engine, the fiber optic sensor is typically placed within a housing mated with a cylinder of the engine. The housing typically has a metal diaphragm that is attached to one end of the fiber optic sensor. Pressures exerted on the diaphragm are transferred to the fiber optic sensor, thereby changing the overall length of the sensor and allowing measurement of continuous real-time in cylinder pressures permitting improved engine control, providing preventive maintenance data, and predictive emissions monitoring. When used to monitor the performance of compressors and pumps, the fiber optic sensor is imbedded within an aluminum alloy rod, or similar metal, by an encasing process. The aluminum rod encasing the fiber optic sensor is then placed within a metal housing having a diaphragm similar to that described above in relation to engine monitoring. By placing the diaphragm in contact with the fluid being transferred by the compressor and/or pump, measurements of cavitation, flow instability, and surge detection are possible, thereby reducing the risk of catastrophic mechanical failure.
As bearing condition sensors, fiber optic sensors are used to monitor the condition of bearing or rotor imbalance. Typically, the fiber optic sensor is encased within a housing that includes a deformable diaphragm. The fiber optic sensor is in contact with the diaphragm which is, in turn, in contact with the outer race of a bearing, thereby allowing for the transfer of any vibrations between the associated bearings and the outer race to the fiber optic sensor.
In non-contact proximity sensors, fiber optic sensors are used to measure shaft vibration, rotor thrust position, shaft rotational speed, as well as rotor imbalance and misalignment. In these applications, the fiber optic sensor is encased within a steel rod having a magnet attached to an end thereof. The steel rod encasing the optical fiber and the magnet are positioned within a stationary housing. The housing is then located such that the magnet is in close proximity to the rotating shaft to be monitored. Imbalances in the shaft cause the magnet to move which motion is transferred to the optical sensor for monitoring of the position or condition of the shaft.
As temperature sensors, fiber optic sensors are typically inserted into areas desired to be monitored, or imbedded into cast parts, thereby allowing the direct measurement of temperatures therein.
Typically, fiber optic sensors have been used to monitor systems that allow for stationary or fixed placement of the sensor within the system. The construction of these sensors have made it difficult if not impossible to monitor processes, systems, or machines that require the optical fiber and the associated fiber optic sensor to be moved throughout the system being monitored. Further, these systems typically require the fiber optic sensor to be cast within a part or structure to be monitored, or placed within a housing that is attached directly to the system to be monitored, thereby adding to the size and cost associated with the monitor system.
The manufacturing procedures and processing of optical fibers and fiber optic cables are numerous and varied. Many of these processes include placing a stress on the optical fiber or fibers being processed. These stresses when applied over time, however short, result in sub-critical growth of the pre-existing flaws located within the optical fibers, thereby decreasing the overall strength of the optical fiber. In certain applications, it is important that the optical fiber, or bundle of fibers, has sufficient strength to withstand loads place thereon without damaging the optical fiber or overall fiber optic cable. As a result, reliability models are created to estimate the strength of the fiber and the associated fiber optic cables after the processing and manufacturing. Reliability models for optical fibers are based on three things: the size distribution of flaws or cracks within the fiber; fatigue crack growth parameters; and the stress-time profile which the fiber experiences during processing. High-stress processing events may result in degradation of the fiber strength. Until now, direct measurements of the stresses exerted on an optical fiber during high-speed processing has not been possible, and, as a result, the stress-time profile of optical fiber has been an assumed quantity.
The ability to collect real-time measurements of the stresses exerted on an optical fiber during processing and cable manufacturing would be valuable for reliability analysis and modeling, process and equipment design, trouble-shooting of manufacturing lines, as well as fiber and cable installation.
One aspect of the present invention is to provide a method and system for measuring stress exerted on an optical fiber including providing an optical fiber that includes a fiber optic sensor, and exposing the optical fiber and the fiber optic sensor to various stresses associated with a process by moving the optical fiber and the fiber optic sensor through the process to be measured. The method and system further include transmitting a source light signal through the optical fiber as the optical fiber and the fiber optic sensor are exposed to the various stresses, receiving a return light signal from the fiber optic sensor as the optical fiber and the fiber optic sensor are exposed to the various stresses, and comparing the source light signal to the return light signal for determining the stresses exerted on the optical fiber.
Another aspect of the present invention is to provide a method and a system for measuring stress exerted on an optical fiber including providing a light source emitting a light having a predetermined frequency, providing a first photo detector coupled to the light source that produces a first electrical signal proportional to the light, providing a first adjustable amplifier coupled to the first photo detector which amplifies a first electrical signal therefrom, and providing the optical sensor coupled to the light source by the optical fiber to transmit at least a portion of the light to the optical sensor. The method and system further includes providing a second photo detector coupled to the optical sensor that detects at least a portion of the light reflected from the optical sensor, and produces a second electrical signal proportional thereto, providing a second adjustable amplifier coupled to the second photo detector that amplifies the second electrical signal therefrom, providing a comparator coupled to the second photo detector that compares the first and second electrical signals to provide a signal representative of the relationship between the first and second signals. In one embodiment, a microcontroller is coupled to the comparator and generates a plurality of trigger signals at a fixed frequency, each initiating a modulated cycle and further that generates a control signal in response to detecting a predetermined transition between the first and second output voltages. The method and system further includes providing a modulator coupled to the light source and the microcontroller to modulate the light source in a periodic manner to provide pulses in response to receiving a trigger signal from the microcontroller. A counter is coupled to the microcontroller and begins counting the periodic pulses in response to receiving a trigger signal, and ends counting in response to receiving a control signal from the microcontroller to generate a count value. The optical fiber and the fiber optic sensor is exposed to various stresses associated with a process by moving the optical fiber and the fiber optic sensor through the process to be measured, and the microcontroller computes the stresses exerted on the optical fiber in response to receiving the count value for each modulation cycle.
Yet another aspect of the present invention is to provide a method for determining the amount of stress exerted on an optical fiber that utilizes a source of light signals, a detector for detecting a return light signal, a comparator for comparing the emitted light signal to the return light signal, and a microcontroller for calculating the stress exerted on the optical fiber. The system includes providing an optical fiber, with a fiber optic sensor. The optical fiber and the fiber optic sensor is exposed to various stresses associated with a process by moving the optical fiber and the associated fiber optic sensors through the process to be measured, while monitoring the detected light signals for calculating the stress exerted on the optical fiber during the process.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.